A magnetic read head retrieves magnetically encoded information that is stored on a magnetic medium or disc. The magnetic read head is typically formed of several layers that include a top shield, a bottom shield, and a read element or sensor positioned between the top and bottom shield. The read element may be a magnetoresistive device, such as a magnetoresistive (MR) sensor, a giant magnetoresistive (GMR) sensor, or a tunneling magnetoresistive (TMR) sensor. The read element includes at least one layer of a ferromagnetic material, which is magnetized along an easy axis of low coercivity. The read element may be oriented such that the easy axis is transverse to the direction of disc rotation and parallel to the plane of the disc. Magnetic flux from the disc surface causes rotation of the magnetization vector of the ferromagnetic layer of the read element, which in turn, causes a change in electrical resistivity of the read element. The change in resistivity of the read element can be detected by passing a sense current through the read element and measuring a voltage across the read element. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary.
During a read operation, the top and bottom shields ensure that the read element reads only the information stored directly beneath it on a specific track of the magnetic medium or disc by absorbing any stray magnetic fields emanating from adjacent tracks and transitions. A plurality of magnetic domains exist within the bottom shield. The magnetic domains are separated by one of a plurality of magnetic domain walls. Each domain has a magnetization that is oriented in a direction different from the magnetization of all adjacent domains. The application of an external magnetic field either during manufacture or from an adjacent track or transition of the magnetic storage medium during operation to a shield can cause the magnetization of each of the domains within that shield to rotate, thereby causing the domains to move, grow, or shrink. Thus, the domain walls are relocated due to the external magnetic field. The movement of a domain wall through a portion of the shield that is directly adjacent the read element results in Barkhausen noise, which is a local perturbation of the magnetic structure within the read element, producing an unwanted change in the resistance of the read element. Until recently, Barkhausen noise induced by domain wall movement has been negligible. However, as storage densities on magnetic media and discs have increased, the read element has necessarily become smaller, more sensitive, and more susceptible to Barkhausen noise created by domain wall movement.